The present invention relates to the driving mechanism of an automatic seat belt system and in particular to a drive device of the type that employs a drive tape.
In many specific forms of known automatic seat belt systems (also sometimes called passive seat belt systems) the upper outboard end of a shoulder belt is moved along a guide rail installed along the edge of the vehicle roof between a rearward position in which the belt extends across the vehicle occupant's torso in a restraining configuration and a forward position in which the belt is located away from the occupant to enable him or her to enter or leave the vehicle. In a known form of drive device for automatic seat belt systems, a thin drive tape, such as a metal strip coated with a flexible polymeric material, is driven by a motor-driven drive mechanism installed at a suitable location remote from the guide rail. The drive tape is guided from the drive mechanism to the rearward end of the guide rail by a cylindrical casing. The guide rail itself includes a guideway for the drive tape that maintains it in position within the guide rail. An example of an automatic seat belt system of the type to which the present invention relates is found in the present applicant's U.S. Pat. No. 4,498,690 issued Feb. 12, 1985.
It is advantageous for the portion of the drive tape that moves along the guide rail to be positioned flatwise with respect to the edge of the vehicle roof, thereby to minimize the projection of the guide rail from the roof into the passenger compartment. On the other hand it is desirable for the drive device, which includes a drive sprocket or reel, to be arranged such that the sprocket or reel lies flatwise to the side of the vehicle, again to minimize the widthwise dimension of the space for the drive device. The orientation of the drive sprocket or reel for the tapes means that the tape lies edgewise of the side of the vehicle. Accordingly, the drive tape undergoes a twist along the path between the drive device and the guide rail. Moreover, the casing is not straight, so the tape twists along the path through the casing. For this reason it is common to provide a cylindrical tubular casing between the drive device and the guide rail in order to constrain the tape to move along the prescribed path and distance and at the same time to permit it to twist. In order to minimize the frictional resistance to movement of the tape through the casing, it is necessary to leave a clearance between the edges of the tape and the walls of the casing. In other words the diameter of the casing is slightly larger than the width of the tape. The provision of such a clearance, however, does not entirely solve the problem of high resistance to movement of the tape through the casing, inasmuch as the tape tends assume a zig-zag or tortuous path along the casing when it is driven in a direction to push the belt transfer anchor along the guide rail from the rearward, restraining position to the forward, releasing position. The zig-zag path of the belt along the casing increases the frictional resistance to the movement of the tape and also produces a bothersome noise as the configuration of the buckles and twists along the tape change in the course of its movement.
One solution to the problems with cylindrical casings has been to provide a premolded casing arranged to conform to the desired path between the drive device and guide rail. Such premolded casings eliminate the play in the drive tape and also eliminate the noise problem. On the other hand, the resistance to sliding of the tape member along the molded casing increases considerably, thus requiring that the power of the drive motor be correspondingly increased. The greater forces involved in the system can also result in breakage of the tape.